The present invention generally relates to methods and apparatus for manufacturing serpentine fluorescent tubes, and more particularly concerns a method and apparatus for coating a fluorescent tube with a phosphor after the tube has been bent into a serpentine shape.
In today's aviation industry, avionics engineers are involved in continuing quest to improve the optical performance of avionics displays. One particular area of concern is fluorescent tubes for backlighting liquid crystal displays.
Typically, fluorescent lamps utilized in the avionics industry are serpentine and are constructed by coating the interior of transparent linear glass tubes with a fluorescent phosphor substance. The linear tubes are then later heated to the working temperature of the glass and are bent into the desired serpentine shape.
Another method to fabricate serpentine fluorescent lamps has been to bend transparent glass tubes into a "U" shape prior to coating with phosphors from the same type of phosphor slurry used to coat linear tubes. Success has been claimed for uniform deposition of phosphors from slurries onto "U" shaped tubing, but "S" shaped or "M" shaped tubes have not been uniformly coated with phosphor by the typical flush coat slurry method. In order to make "S" or "M" shaped tubes, it has been attempted to weld together 2 or 3 "U" shaped phosphor slurry coated tubes to create "S" and "M" shaped lamps respectively.
While these methods have been used widely in the past, all existing methods of fabricating serpentine lamps have several serious draw backs. First of all, in bend-after-coat schemes, the efficiency of the phosphors is diminished when they are heated to a temperature sufficient to allow bending of the tube. Secondly, the bending of the tube results in lacerations or cracks in the phosphor coating. This results in a diminution in luminance uniformity and chromaticity uniformity, as well as absolute luminance per unit area.
In the method involving welding several slurry coated "U" shaped tubes together, the areas where the "welding" occurs are exposed to high temperatures and the phosphor therein are degraded as a result.
Consequently, there exists a need for improved manufacture of fluorescent tubes for use in the avionics industry, in which phosphor efficiency and uniformity of luminance and chromaticity are not degraded as a result of the fabrication process.